test(opencode): verify fork rollback

[LeXwDeX]

Summary

• inject a deterministic real-SQLite failure on the second copied part after three nested publications complete
• verify copied durable events and message/part projections roll back while the source remains unchanged and Session Created may remain
• record batch B ticket status and evidence; production code is unchanged

Verification

• cd packages/opencode && bun test test/session/fork-batch.test.ts — 3 pass, 0 fail, 52 expect
• cd packages/opencode && bun typecheck — passed
• mutation check: removing the outer copy transaction leaves two target messages and makes the new test fail; mutation was reverted
• commit and pre-push hooks: lint 4848 warnings / 0 errors, full typecheck 29/29